Re: [PATCH v3] rcu: ftrace: avoid tracing a few functions executed in stop machine

Paul E. McKenney
2022-04-26T16:15:20Z

On Tue, Apr 26, 2022 at 11:22:50AM -0400, Steven Rostedt wrote:
> On Tue, 26 Apr 2022 18:55:11 +0800
> Patrick Wang <patrick.wang.shcn@gmail.com> wrote:
>
> > Because of the change of stop machine implementation, there are functions
> > being called while waiting currently:
> >
> > ----------------------------------------------------------------
> > Former stop machine wait loop:
> > do {
> > cpu_relax(); => macro
> > ...
> > } while (curstate != STOPMACHINE_EXIT);
> > -----------------------------------------------------------------
> > Current stop machine wait loop:
> > do {
> > stop_machine_yield(cpumask); => function (notraced)
> > ...
> > touch_nmi_watchdog(); => function (notraced, inside calls also notraced)
> > ...
> > rcu_momentary_dyntick_idle(); => function (notraced, inside calls traced)
> > } while (curstate != MULTI_STOP_EXIT);
> > ------------------------------------------------------------------
> >
> > These functions (including the calls inside) should be marked notrace to avoid
> > their codes being updated when they are being called. The calls inside
> > rcu_momentary_dyntick_idle() still remain traced, and will cause crash:
> >
> > rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
> > rcu: 1-...!: (0 ticks this GP) idle=14f/1/0x4000000000000000 softirq=3397/3397 fqs=0
> > rcu: 3-...!: (0 ticks this GP) idle=ee9/1/0x4000000000000000 softirq=5168/5168 fqs=0
> > (detected by 0, t=8137 jiffies, g=5889, q=2 ncpus=4)
> > Task dump for CPU 1:
> > task:migration/1 state:R running task stack: 0 pid: 19 ppid: 2 flags:0x00000000
> > Stopper: multi_cpu_stop+0x0/0x18c <- stop_machine_cpuslocked+0x128/0x174
> > Call Trace:
> > Task dump for CPU 3:
> > task:migration/3 state:R running task stack: 0 pid: 29 ppid: 2 flags:0x00000000
> > Stopper: multi_cpu_stop+0x0/0x18c <- stop_machine_cpuslocked+0x128/0x174
> > Call Trace:
> > rcu: rcu_preempt kthread timer wakeup didn't happen for 8136 jiffies! g5889 f0x0 RCU_GP_WAIT_FQS(5) ->state=0x402
> > rcu: Possible timer handling issue on cpu=2 timer-softirq=594
> > rcu: rcu_preempt kthread starved for 8137 jiffies! g5889 f0x0 RCU_GP_WAIT_FQS(5) ->state=0x402 ->cpu=2
> > rcu: Unless rcu_preempt kthread gets sufficient CPU time, OOM is now expected behavior.
> > rcu: RCU grace-period kthread stack dump:
> > task:rcu_preempt state:I stack: 0 pid: 14 ppid: 2 flags:0x00000000
> > Call Trace:
> > schedule+0x56/0xc2
> > schedule_timeout+0x82/0x184
> > rcu_gp_fqs_loop+0x19a/0x318
> > rcu_gp_kthread+0x11a/0x140
> > kthread+0xee/0x118
> > ret_from_exception+0x0/0x14
> > rcu: Stack dump where RCU GP kthread last ran:
> > Task dump for CPU 2:
> > task:migration/2 state:R running task stack: 0 pid: 24 ppid: 2 flags:0x00000000
> > Stopper: multi_cpu_stop+0x0/0x18c <- stop_machine_cpuslocked+0x128/0x174
> > Call Trace:
> >
> > Mark the calls inside rcu_momentary_dyntick_idle():
> > rcu_preempt_deferred_qs()
> > rcu_preempt_need_deferred_qs()
> > rcu_preempt_deferred_qs_irqrestore()
> > as notrace to prevent this.
> >
> > Signed-off-by: Patrick Wang <patrick.wang.shcn@gmail.com>
> > ---
> > v1->v2:
> > - Modify log message.
>
> Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>

I have reverted Patrick's earlier commit and replaced it with this
new one, including Steve's ack. Thank you both!

Thanx, Paul